Rat liver carcinogenesis has been shown to be sex differentiated in many different models and gonadal as well as pituitary hormones have been shown to influence the carcinogenic process. Our investigations indicate that the effects of gonadal hormones on rat liver carcinogenesis in some systems are mediated via an influence on the hypothalamo-pituitary-liver axis mediated by the sex differentiated pattern of growth hormone secretion. The specific aims of the present study is to use rat liver carcinogenesis as a tool to further investigate how sex hormones, directly or via the hypothalamo-pituitary liver axis, act as modifiers of the carcinogenic process. The sexual dimorphism observed at different stages of hepatocarcinogenesis will also be used to identify molecular mechanisms involved in the stepwise loss of control of proliferation and differentiation during the process towards a malignant tumor. Hopefully, sex differentiated growth of preneoplastic lesions can serve as a tool to identify the genes central for the observed dimorphism in proliferation and help finding common levels of regulation in these lesions. The following questions will be addressed: 1. Are estrogens promoters of liver carcinogenesis in female rats or are the previously observed effects secondary, due to an influence on prolactin secretion? - 2. Is the promotive effects of compounds that are more efficient in female that in male rats due to a direct effect of gonadal hormones or are the effects mediated via the hypothalamo-pituitary-liver axis, in analogy with previous results concerning male-specific promoters? - 3. Which is the significance of the hormonal environment during tumor progression as a determinant of the incidence and latency period for hepatoma development? - 4. Does the loss of sexual differentiation of various liver functions in male and female liver nodules indicate a withdrawal from the normal endocrine control of rat liver. - 5. Could a changed expression of liver-specific transcription factor(s) act as a common denominator of the altered genetic program seen in both male and female nodules?. Are these transcription factors influenced by hormones per se? - 6. Can any parameters, e.g., changed regulation of genes associated with hepatocyte proliferation or differentiation, be identified that are consistently, in different models, associated with a shorter latency period for tumor formation in rats of either sex? - 7. Is the observed increase in c-myc expression that has been shown to be connected with a higher promotion efficiency in males during treatment with several different regimens a general feature in sex differentiated promotion? - 8. Could c-myc, expressed as an externally controlled transgene in initiated rats, substitute for promotion and/or affect the latency period for tumor development? The present project, combining investigations of the role of different levels of endocrine control of carcinogenesis in a multistep model with molecular analysis of growth control, represents a unique approach to enlighten the mechanisms by which hormones affect the carcinogenic process.